A problem in the use of robots in tooling operations is the problem of overload on the robot arm. This may occur if the robot arm or tool collides with something or if the tool gets stuck in the workpiece. Such an overload can cause serious damage to the robot, the robot arm, the tool or to the workpiece.
Several devices have been proposed and are in use that detect stresses and overloads on the robot arm and immediately shut down the robot when overload is detected in order to avoid damage to the robot or to the workpiece. These devices include expandable breakaway joints and connectors that break away when the robot arm experiences an overload. These are troublesome because they often cause repetitive downtime and require reprogramming of the robot. Other devices include safety joints and couplings having spring-loaded elements whose separation is sensed by proximity detectors. These devices usually trigger an emergency stop when an overload is encountered. The sensitivity of the prior art couplers can be adjustable only by manually varying spring load, such as, for example, by turning a nut to increase the bias on the coupler's springs.
A major problem with all of the known devices is that they are not adjustable "on the fly" under program control. "On the fly" adjustability is required to allow for varying degrees of overload sensitivity throughout a programmed motion of the robot. When the robot is moving rapidly about the work area the device needs to be less sensitive in order to avoid inadvertent stoppage of the robot. Similarly, the device should be more sensitive when the robot is performing slower or more intricate motions. Also, the device should allow for interchangeable tooling and ready adjustment for varying weights of tools.